1. Field of the Invention
The present invention generally relates to lighting of billboards, signs, buildings, and other structures and, more particularly, relates to front illumination of surfaces using light emitting diodes (LEDs).
2. Description of the Related Art
Historically, light fixtures providing front illumination of large advertising billboards have utilized various types of illumination sources, including electro-florescent tubes and flood lights (incandescent, high energy discharge, projection lamps). The illumination source is enclosed in a fixture which is mounted on an extended arm, and the extended arm positions the illumination source, also referred to as a light source, far enough away from the billboard face to illuminate the image area (or “copy area”) for viewing in low light conditions. Typically, because of the beamwidth limitations of the light source, two or more light fixtures are needed to illuminate the copy area. The number of light fixtures needed thus depends upon the height and width of the copy area and the beamwidth of the light source. Also, in order to provide the desired dispersion of the light from the light sources, and to provide for relatively even illumination from top to bottom of the copy area, the light sources have to be set back from the billboard face, typically 6 to 8 feet. The fixture mounting system is typically attached to the billboard support structure, either at or near the top of, the bottom of, or both the top and the bottom of, the billboard face. The fixture mounting system therefore typically extends the light fixture in front of the billboard face. The mounting device for each fixture typically consists of a metal tube, arm, or conduit which is mounted to the billboard structure, and which extends out approximately 6 to 8 feet, the light source typically being attached at or near the end of the fixture.
A larger number of light sources provides for more even illumination of the copy area, but generally results in higher costs, more physical structures, higher wind loading, and more maintenance issues. Conversely, a lower number of light sources results in lower cost, fewer physical structures, lower wind loading, and fewer maintenance issues, but increases the likelihood that there will be noticeable and undesirable differences between better lit (lighter) areas and more poorly lit (darker) areas. As a compromise between these opposing goals and choices, a typical installation may use only a few light sources, spaced at intervals of 8 to 12 feet. To compensate for using a small number of light sources, the light sources are generally wide-angle light sources, so that the fields of illumination provided by the light sources overlap somewhat, thereby reducing the contrast between the differently-illuminated areas. The use of wide angle light sources, however, brings on other problems such as, but not limited to, (1) inefficiency because the use of wide-angle light sources causes a substantial amount of the light emitted to be spread outside the borders of the copy area; (2) undesired heat generation because more light output, and therefore more energy, is required so that the light which strikes the copy area is of sufficient intensity to properly illuminate the copy area; and (3) light pollution problems because the light which does not strike the copy area contributes to night sky light pollution. Night sky pollution has become such a severe problem in many large cities that only the brightest stars are visible at night.
Simply replacing the current light sources with LED light sources would seem to be a solution but, in reality, is not a solution because of several problems: (1) a large number of LEDs is required to provide the desired illumination, (2) the optics and lensing required to illuminate the copy area sufficiently is very complex, (3) the heat generated by a large number of LEDs in close proximity or clustered together causes the LEDs to quickly overheat and fail, and (4) cost, windloading, esthetic, maintenance, and other considerations. Thus, neither simply replacing conventional light sources with clustered LEDs, nor adding more projecting fixtures with fewer LEDs in a fixture, are viable options.